Tube-in-tube threaded dashpot end plug

ABSTRACT

A nuclear fuel assembly having a tube-in-tube control rod guide tube design that incorporates an end plug that extends axially upward to an elevation above the lower most grid where it is sealed at its upper end to the lower end of the control rod guide tube. The guide tube lower end plug has a threaded recess in its upper surface that mates with a corresponding dashpot end plug threaded extension that is formed as an insert in the lower end of the guide tube. A hole formed through the outer wall of the guide tube end plug at the elevation of the lower portion of the recess provides a positive inspection port for assuring the proper seating of the dashpot. A method of manufacture of such a fuel assembly is also disclosed.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a divisional application based on U.S. Ser.No. 11/583,466, filed Oct. 18, 2006

BACKGROUND OF THE INVENTION

1. Field of the Invention This invention relates in general to nuclearreactor fuel assemblies and more particularly to control rod guidethimble designs for use within nuclear reactor fuel assemblies.

2. Related Art

In nuclear reactors of the type designed in the former Soviet Union, thereactor core is comprised of a large number of elongated fuelassemblies, each having a plurality of fuel rods held in an organizedhexagonal array by a plurality of grids spaced longitudinally along thefuel rods and secured to stainless steel control rod guide thimbles. Thestainless steel control rod guide thimbles extend above and below theends of the fuel rods and are attached to the top and bottom nozzles,respectively. The fuel assemblies are arranged in the reactor vesselwith the bottom nozzles resting on a lower core plate. An upper coreplate rests on the top nozzles.

The top nozzles in the Soviet design are non-removably fixed to thestainless steel control rod guide thimbles of the fuel assembly. Thesecomplex nozzles perform several functions. First, they position the rodcontrol cluster assembly (RCCA) relative to the control rod guidethimbles within the core so the position of the RCCA relative to theupper core plate is fixed. The RCCA positions the control rods, whichare inserted into the fuel assembly as a group or cluster.

The Soviet nozzle also dampens the velocity of the control rods usingsprings to remove energy when the RCCA rods are dropped into the reactorcore during an emergency shutdown, commonly known as a “scram”. Thenozzle also supplies spring loads for supporting the internals. When theupper core plate is lowered onto the nozzles, it compresses the nozzlespring. In addition, the Soviet nozzle is designed to protect thecontrol rods when the fuel assembly is removed from the reactor vessel.Under these conditions, the RCCA is at or below the top edge of thenozzle. Finally, the Soviet design of the top nozzle allows the fuelassembly to be handled when lifted out of the core by transferring theloads through the nozzle.

Thus, the Soviet nozzle is designed to function in two positions, freeand compressed. As stainless steel is used for the thimbles of theSoviet fuel assembly, the relative separation between the interior ofthe reactor vessel and the fuel assemblies remains constant once theassembly is in position. Spring loads are such that the nozzles cansupport the internals, and the spring loads as well as the RCCApositions are fixed so that all functions are static. As a result, thenozzle has built-in references around which the internals are designed.The stainless steel thimbles used in the Soviet design impose higherreactivity cost on the fuel assemblies due to their neutron capturecross-section, i.e., neutron absorption rate, and they are moredifficult to attach to the grids of the fuel assemblies. Non-Soviet fuelassemblies utilize zircaloy for the thimbles which imposes lessreactivity cost. However, zircaloy has a different coefficient thermalexpansion than the stainless steel reactor vessel, and grows duringirradiation. Expandable top nozzles, which accommodate for thesevariations in the dimensions of the different components within thereactor are disclosed in, for example, U.S. Pat. Nos. 4,534,933;4,687,619; 4,702,882 and 4,986,959. Such nozzles, however, are used inreactors in which the top core plate rests on a core support in the formof a circular ledge within the reactor vessel. In the Soviet typereactor, the core plate rests on and is supported by the top nozzles.

As mentioned, the Soviet design top nozzle is permanently attached tothe thimble tubes of the fuel assembly. The above-mentioned patentsdisclose removable top nozzles and U.S. Pat. No. 5,479,464 took thattechnology to another step in applying the removable top nozzles to theSoviet type reactor nozzle design. However, the substitution of zircaloyfor stainless steel in some of the fuel assembly components, such as thethimble tubes in which the control rods move, requires furthermodifications to assure that impact loads experienced by the assembliescan be absorbed without damaging the assemblies or other corecomponents. For example, in the VVER 1000 type Soviet designed reactor,when the control rods scram, they freefall and impact the top nozzle ata very high velocity. This fuel design does not use a dashpot or anyother hydraulic mechanical device to minimize these high impacts. In thedesign described in U.S. Pat. No. 5,479,464, springs are employed toabsorb some of this load. However, further means are desired to absorbthe shock of the load as well as the load itself. In a standard westernfuel assembly design, approximately two feet before full insertion ofthe control rods into the fuel assembly, the tips of the control rodsenter a small diameter portion of the thimble tube called the dashpot.This dashpot is approximately one (1) millimeter larger than the controlrods. Because the control rods are moving very fast at this point in thescram, there is a large volume of water which has to be accelerated uppast the falling control rods to make room for them in the dashpot. Thisprocess causes the control rods to decelerate rapidly, thus lesseningthe impact velocity of the control rod assembly at the top nozzleadapter plate. The standard VVER 1000 style fuel assemblies do not havea dashpot and therefore the control rod assembly impacts the top nozzleat a much higher velocity. As the kinetic energy is equal to one halfthe mass×the velocity², if the velocity at impact on the VVER 1000 fueldesign is four times that of the standard western pressurized waterreactor design, then the total energy which has to be absorbed afterimpact is sixteen (16) times as much. A new high energy absorption topnozzle has been designed to absorb that energy and is described in U.S.Pat. No. 6,738,447. This high energy absorption top nozzle assures thatthe impact loads expected during scram events will be absorbed withoutdamaging the nozzle, fuel assembly and/or control rod assembly. Inaddition, this new design is expandable to accommodate expansion andgrowth of the zircaloy components of the fuel assembly while supportingthe upper core plate in a fixed position. More recently, the TemelinUnit 1 and 2 reactors of the VVER 1000 type design have experienced someproblems associated with incomplete control rod insertions, which raisessome safety concerns.

Accordingly, there is a further need for an improved VVER 1000 type offuel assembly design that will overcome the incomplete control rodinsertion problem while accommodating zircaloy clad control rod thimbletubes.

SUMMARY OF THE INVENTION

The aforegoing needs are satisfied by a new nuclear fuel assembly designhaving a top nozzle, a bottom nozzle and a plurality of elongatedthimble tubes having an axial dimension. The thimble tubes are supportedin a parallel, spaced array extending axially between the top nozzle andthe bottom nozzle. A plurality of spacer grids are arranged in tandembetween the top nozzle and the bottom nozzle, supporting the thimbletubes in this parallel array at spaced axial elevations between the topnozzle and the bottom nozzle. A bottom thimble end plug extends axiallyfrom the bottom nozzle towards the top nozzle and terminates at ajuncture with a lower most portion of a corresponding thimble tube. Thelower portion of the bottom thimble end plug is secured to the bottomnozzle. In one preferred embodiment a dashpot, having an outer diameterthat closely approximates an interior diameter of the thimble tube, isinserted into the lower portion of the thimble tube. The dashpotincludes a lowermost extension having one of either a male or a femalemechanically coupling contour that meets with the other of either a maleor female mechanically coupling contour in an upper inner surface of thebottom thimble end plug and is secured thereto by the interlockingmechanically coupling contours. Desirably the bottom thimble end plug isprovided with a hole through its wall at an elevation proximate an endof the lower most extension of the dashpot that provides a view of theportion of the end of the dashpot lower most extension to confirm theproper seating of the dashpot within the control rod thimble tube.

In one preferred embodiment the bottom thimble end plug extends axiallyfrom the bottom nozzle towards the top nozzle and substantiallyterminates at a juncture with a lower most portion of the thimble tube,above a lower most spacer grid. Desirably the outside diameter of thebottom thimble end plug is not substantially larger than a diameter ofthe fuel rods at an axial elevation where the fuel rods areultrasonically inspected, to provide clearance for a substantially rigidultrasonic probe to be inserted straight through the fuel assembly.

In one embodiment the male and female mechanically coupling contours ofthe thimble tube end plug and dashpot extension are threaded contours.Preferably, the dashpot extension includes a male thread and the upperinner surface of the thimble tube end plug includes a recess including amating female thread. Desirably the lead portion of the lower mostdashpot extension has a substantially smooth walled beveled pilot thatprotects the mechanically coupling contours during insertion of thelower most extension into the thimble end plug recess.

The invention further provides a method of manufacturing a fuel elementskeleton having a top nozzle, a bottom nozzle, and a plurality ofthimble tubes having an axial dimension. The thimble tubes are supportedin a spaced parallel array extending axially between the top nozzle andthe bottom nozzle. A plurality of spacer grids are arranged in tandembetween the top nozzle and the bottom nozzle to support the thimbletubes in the spaced parallel array at spaced axial elevations betweenthe top nozzle and the bottom nozzle. A bottom thimble end plugextending axially from the bottom nozzle towards the top nozzle,substantially terminates at a juncture with the lower most portion of acorresponding thimble tube. The bottom thimble end plug upper surfacehas a recess in an upper end proximate the juncture of the lower mostportion of the corresponding thimble tube. The recess in the uppersurface of the thimble end plug has one of either a female or malemechanically coupling contour. The thimble tube in its lower endincludes a dashpot within its interior having an outer diameter thatclosely approximates an interior diameter of the thimble tube. Thedashpot includes a lower most extension with the other of either themale or female mechanically coupling contour that mates with themechanically coupling contour in the recess in the upper end of thethimble end plug. The method includes the step of setting up a pluralityof components comprising the top nozzle, bottom nozzle, thimble tubeswith the bottom thimble end plug affixed, and the spacer grids in amanufacturing fixture that positions each of the components in the fuelassembly skeleton. The bottom nozzle is then attached to the bottomthimble end plug employing a standard manufacturing screw. A bulgingtool is inserted into the top of the thimble tube and lowered to anelevation of the lower most grid above the elevation of the thimble endplug. The bulging tool is expanded to press fit the thimble tube to thelower most grid. The bulging tool is then removed from the thimble tubeand the dashpot is inserted into the thimble tube and manipulated toengage the female and male mechanically coupling contours in the thimbletube end plug recess. The bulging tool is again reinserted and expandedto press fit the dashpot to a lower section of the thimble tube and thethimble tube to the plurality of spacer grids at the several elevationsabove the lower most grid. The bulging tool is then removed and thecomponents, now assembled are removed from the manufacturing fixture. Inthe preferred embodiment a hole is provided in a wall of the bottomthimble end plug at an elevation where the dashpot seats in the bottomthimble end plug and includes the step of viewing the seating of thedashpot end plug to ensure its proper engagement.

BRIEF DESCRIPTION OF THE DRAWINGS

A further understanding of the invention can be gained from thefollowing description of the preferred embodiments when read inconjunction with the accompanying drawings in which:

FIG. 1 is a planned view partially in section of a rectangular fuelassembly with guide thimbles that can incorporate the design of thisinvention;

FIG. 2 is a planned view of a hexagonal fuel assembly with guidethimbles that can incorporate the design of this invention;

FIG. 3 is a planned view partially in section of a guide thimble designof this invention;

FIG. 4 is a schematic illustration of the dashpot end plug beingremotely aligned and inserted into the recess in the upper surface ofthe control rod guide tube lower end cap; and

FIG. 5 is a schematic cross section illustrating the relative halfpattern of control rods and fuel rods within an exemplary fuel assembly.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a typical nuclear fuel assembly 10 for a pressurizedwater reactor that can employ the control rod guide thimbles of thisinvention to slow down the control rods when they are dropped into thereactor core under a scram condition. FIG. 1 shows an elevational viewof a nuclear reactor fuel assembly, represented in vertically shortenform and being generally designated by reference character 10. The fuelassembly 10 has a structural skeleton which, at its lower end includesthe bottom nozzle 14. The bottom nozzle 14 supports the fuel assembly 10on a lower core support plate 18 in the core region of the nuclearreactor (not shown). In addition to the bottom nozzle 14, the structuralskeleton of the fuel assembly 10 also includes a top nozzle 12 at itsupper end and a number of guide tubes or thimbles 20, which extendlongitudinally between the bottom and top nozzles 14 and 12 and at theopposite ends are rigidly attached thereto.

The structural skeleton of the fuel assembly 10 further includes aplurality of traverse grids 22, that are axially spaced along, andmounted to, the guide thimble tubes 20. In the final assembly the gridsfunction to maintain an organized array of elongated fuel rods 24traversely spaced and supported by the grids 22. Also, the structuralskeleton of the fuel assembly 10 includes an instrumentation tube 16located in the center thereof, which extends and is captured between thebottom and top nozzles 14 and 12. With such an arrangement of parts,fuel assembly 20 forms an integral unit capable of being convenientlyhandled without damaging the assembled parts.

The fuel rods 24 are not actually part of the structural skeleton of thefuel assembly 10, but are inserted, respectively, in the individualcells within the grids 22 before the top nozzle is finally affixed atthe end of fuel assembly manufacture. As mentioned above, the fuel rods24, as in the array shown in the fuel assembly 10, are held in spacerelationship with one another by the grids 22 spaced in tandem along thefuel assembly length. Each fuel rod 24 includes a stack of nuclear fuelpellets 26 and is closed at its opposite ends by upper and lower fuelrod end plugs 28 and 30. The pellets 26 are maintained in the stack byplenum spring 32 disposed between the upper end plug 28 and the top ofthe pellet stack. The fuel pellets 26, composed of fissile material, areresponsible for creating the thermal power of the reactor. A liquidmoderator/coolant such as water or water containing boron, is pumpedupwardly through a plurality of flow openings in the lower core supportplate 18 to the fuel assembly 10. The bottom nozzle 14 of the fuelassembly 10 passes the coolant upwardly through the guide tubes 20 andalong the fuel rods 24 of the assembly 10 in order to extract heatgenerated therein for the production of useful work. For the purpose ofillustration, FIG. 1 shows a 17×17 array of fuel rods 24 in a squareconfiguration, it should be appreciated that other arrays of differentdesigns and geometries are employed in various models of pressurizedreactors. For example VVER fuel assembly is formed in a hexagonal arrayas shown in FIG. 2, however, the basic components of the structuralskeleton are the same as that illustrated in FIG. 1. Like referencecharacters are employed to identify corresponding components shown inFIG. 1 and FIG. 2.

To control the fission process, a number of control rods 34 arereciprocally movable in the guide thimbles 20 located at predeterminedpositions in the fuel assembly 10. A rod cluster control mechanism 36positioned above the top nozzle 12 supports the control rod 34. Thecontrol mechanism has an internally threaded cylindrical member 38 whichfunctions as a drive rod with, a plurality of radial extending flukes orarms 40. Each arm 40 is interconnected to a control rod 34 such that thecontrol rod mechanism 36 is operable to move the control rods verticallyin the guide thimbles 20 to thereby control the fission process in thefuel assembly 10, all in a well known manner.

The grids 22 are mechanically attached to the control rod guide thimbles20 and the instrumentation tube 16 by welding, or preferably by bulging.Bulging is particularly desirable where welding dissimilar materials isdifficult.

FIG. 3 is a planned view of the lower portion of a control rod guidethimble tube 20 which is closed at its lower end by an extended guidethimble end plug 50. The guide thimble end plug 50 is attached to thebottom nozzle 14 by a threaded thimble screw 56 which is insertedthrough a bore 58 that is countersunk in the bottom nozzle 14. Thethimble screw 56 attaches to a threaded recess 60 in the lower end ofthe guide thimble end plug 50. The lower most grid 52 has a sleeveinsert 68, through which the control rod thimble end plug passes. Thesleeve insert 68 is affixed to the grid straps by welding or brazing andhas an inner annular lip 70 at its lower end that is captured betweenthe control rod thimble end plug 50 and the bottom nozzle 14 by thebottom nozzle thimble screw 56 and supports the lower grid in position.The upper portion of the guide thimble end plug 50 is attached to thelower end of the guide thimble tube 20 and includes a partially threadedrecess 64 in its upper surface. Furthermore, in accordance with thisinvention, the lower portion of the interior of the guide thimble tube20 is narrowed by a dashpot insert tube 44 that has a tapered extendedend 46 that is threaded in an upper portion of the extension to matewith corresponding threads in the guide thimble end plug recess 64. Thelower extension 46 of the dashpot 42 further includes a chamfered smoothlower end that's used to remotely guide the extended end 46 of thedashpot tube 42 into the recess 64. A hex key contoured inner surface 48within the interior of the lower extension 46 of the dashpot tube 42 isprovided to aid in manufacture as will be explained hereafter. One ormore inspection ports 62 are provided within the side of the guidethimble tube end plug 50 at the elevation 62 where the dashpot end plug44 seats in the recess 64 to assure the proper positioning of thedashpot end plug during manufacture. The second lower most gridsurrounding the dashpot 54 is fasten to the thimble tube 20 by bugling.

After irradiation if a fuel assembly is known to have leaking fuel it isdesirable to ultrasonic nondestructively inspect all of the fuel rods todetermine which fuel rod may be leaking. To accomplish this inspectionultrasonic probes are inserted through the rows of fuel rods and controlrod guide thimbles to inspect each of the rods. This is accomplished atan elevation along the fuel assembly just above the lower most grid.However, to accommodate the width of the control rods and provideadequate clearance between the inner walls of the dashpot 42 the outerdiameter of the thimble tube walls 20 will necessarily have to be largerthan the width of the fuel rods and will inhibit the insertion of theultrasonic probes. Reference character 66 in FIG. 5 figurativelyillustrates the circuitous path the ultrasonic probes would have tofollow if the control rod thimbles extend to the elevation of the lowermost grid. To overcome this obstacle the guide thimble end plug 50 hasbeen designed to have an extended axial dimension that terminates justbelow the second lower most grid 54. The guide thimble end plug 50 has awidth only slightly larger than the fuel rods 24 and thus enablespassage of the ultrasonic probes through the rows of fuel rods for thenondestructive examination process to proceed.

The tube-in-tube dashpot design of this invention enables an improvedmanufacturing process that reduces the number of manufacturing stepsrequired, avoids potential manufacturing errors and enables reduction ofscrap rates and ease of assembly.

Previously, the manufacturing process required the set up of allcomponents in a skeleton fixture. Next, special type one (1)manufacturing screws attached the bottom nozzle 14 to the skeletonthimbles 20. A bulging tool was then inserted all the way to the bottomof the skeleton and the bottom grid 52 was bulged in its location. Thebulging tooling was then withdrawn from the assembly 10. The type 1manufacturing screws were then removed and the dashpot assembliesinserted into each thimble 20. Special type two (2) manufacturing screwswere then inserted through the bottom nozzle 14 and the bottom end plug50 of the thimble 20 and into the dashpot end plug and torqued down toform a press fit between the dashpot and the thimble end plug. Thebulging tool was then reinserted into the thimbles and the remainder ofthe bulges were made. The bulging tooling was then withdrawn from theassembly, the type 2 manufacturing screws were removed and a dedicatedgauge was inserted through the bottom nozzle into each thimble end plugto measure the gap between the dashpots and thimble end plugs. Thisspacing cannot exceed a specified value. If the spacing has beenexceeded, the skeleton should be scrapped. Standard product thimblescrews are then inserted and tightened to the required torque prior toremoval of the skeleton from the fixture.

The design of this invention enables a more efficient and cost effectivemanufacturing process. The set up of all components on the skeletonfixture is still required. However, standard product thimble screws 58attach the bottom nozzle to the skeleton control rod thimbles. The lowermost grid 52 can be attached to the guide thimble end plug 50 bywelding. The bulging tool is then inserted all the way to the bottom ofthe guide thimble tube and the second lower most grid adjacent the lowerend of the guide tube 20 is bulged in its location. The bulging tool isthen withdrawn from the assembly and the dashpot assembly 42 is theninserted into each thimble and torqued into position. The axial positionof the dashpot is viewed/gauged through a small hole 62 in the guidethimble end plug. The bulge tooling is reinserted into the thimbles andthe remainder of the bulges are made. The bulge tooling is then removedfrom the assembly prior to the removal of the skeleton from the fixture.

The dashpot end plug 64 has a broached hex 48 in its top end whichallows engagement of a hex wrench for proper torqueing. Although itshould be appreciated that other mating tooling configurations can beemployed to accomplished the same results. For instance, the hex orother articulated contoured surface can be formed in a raised pedestalon the top surface on the dashpot end plug 64 and mate with a toolingsocket having a female recess with a corresponding articulated contouredsurface. The threaded end of the dashpot plug 64 has a non threadedpilot chamfered end which protects the threads during insertion andprovides for remote insertion into the mating female threads in therecess of the top surface of the guide thimble end plug 50. Appropriatechamfers as shown in FIG. 4, threads and the mating guide thimble endplug recess assure no thread damage during assembly and first time everytime proper thread engagement.

The tube-in-tube threaded dashpot end plug provides improvements in thestructural capability of the joint and ease of assembly in themanufacturing process as well as decreases the probability of needingrework or even scraping any of the components as the proper dashpotseating is proven before the dashpot is bulged in the skeleton. Thenumber of manufacturing steps eliminated in this design provides a timesavings as well as a reduction in the number of errors that can occurduring the manufacturing processes.

While specific embodiments of the invention have been described indetail, it will be appreciated by those skilled in the art that variousmodifications and alternatives to those details can be developed inlight of the overall teachings of the disclosure. Accordingly, theparticular embodiments disclosed are meant to be illustrative only andnot limiting as to the scope of the invention which is to be given thefull breath of the appended claims and any and all equivalents thereof.

1. A method of manufacturing a fuel element skeleton having a topnozzle, a bottom nozzle, a plurality of elongated thimble tubes havingan axial dimension, the thimble tubes being supported in a spacedparallel array extending axially between the top nozzle and the bottomnozzle, a plurality of spacer grids arranged in tandem between the topnozzle and the bottom nozzle, supporting the thimble tubes in the spacedparallel array at spaced axial elevations between the top nozzle andbottom nozzle and a bottom thimble end plug extending axially from thebottom nozzle towards the top nozzle and substantially terminating at ajuncture with a lowermost portion of one of the plurality of thimbletubes, wherein the bottom thimble end plug has a recess in an upper endproximate the juncture of the lowermost portion of the one of theplurality of thimble tubes, the recess in the upper end of the thimbleend plug having one of either a female or male mechanically couplingcontour and further including a dashpot having an outer diameter thatclosely approximates an interior diameter of the thimble, the dashpotincluding a lowermost extension with the other of either the male orfemale mechanically coupling contour that mates with the mechanicallycoupling contour in the recess in the upper end of the thimble end plug,comprising the steps of: setting up a plurality of components comprisingthe top nozzle, bottom nozzle, thimble tubes with the bottom thimble endplug affixed and the spacer grids in a manufacturing fixture thatpositions each of the components in the fuel assembly skeleton;attaching the bottom nozzle to the bottom thimble end plug; inserting abulging tool into a top of the thimble tube; lowering the bulging toolwithin the thimble tube to an elevation of a lower most grid of theplurality of grids above an elevation of the thimble end plug; expandingthe bulging tool to press fit the thimble tube to the lowermost grid;removing the bulging tool from the thimble tube; inserting the dashpotinto the thimble tube; manipulating the dashpot to engage the female andmale mechanically coupling contours in the thimble end plug recess anddashpot end plug lowermost extension; inserting the bulging tool withinthe thimble tool; expanding the bulging tool to press fit the dashpot toa lower section of the thimble tube and the thimble tube to theplurality of spacer grids above the lowermost grid; removing the bulgingtool from the thimble tube; and removing the components now assembledfrom the manufacturing fixture.
 2. The method of manufacturing a fuelelement skeleton of claim 1 wherein the step of attaching the bottomnozzle to the bottom thimble end plug comprises securing the bottomnozzle to the bottom thimble end plug with a standard product thimblescrew.
 3. The method of manufacturing a fuel element skeleton of claim 1wherein the dashpot includes a lower dashpot end plug from which thelowermost extension extends and wherein the manipulating step comprisesturning the dashpot end plug so the female and male mechanicallycoupling contours engage.
 4. The method of manufacturing a fuel elementskeleton of claim 3 wherein the dashpot end plug has an upper surfacewith an articulated contour and the turning step comprises engaging thearticulated contour on the upper surface of the dashpot end plug with anengagement tool having a mating articulated extension that mates withthe articulated contour of the upper surface on the dashpot end plug andusing the engagement tool to torque the dashpot end plug in the recessin the thimble end plug.
 5. The method of manufacturing a fuel elementskeleton of claim 1 including the step of viewing the seating of thedashpot end plug in the recess in the bottom thimble end plug recessthrough a hole in a wall of the bottom thimble end plug.